The present invention relates to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics and, more particularly, to such structures used for the storage and retrieval of digital data.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as magnetic field sensors. Digital data memories are used extensively in digital systems of many kinds including computers and computer systems components, and digital signal processing systems. Such memories can be advantageously based on the storage of digital symbols as alternative states of magnetization in magnetic materials provided in each memory storage cell, the result being memories which use less electrical power and do not lose information upon removals of such electrical power.
Such memory cells, and magnetic field sensors also, can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic states, or magnetic conditions, therein. Such devices may be provided on a surface of a monolithic integrated circuit to provide convenient electrical interconnections between the device and the operating circuitry therefor.
Ferromagnetic thin-film memory cells, for instance, can be made very small and packed very closely together to achieve a significant density of information storage, particularly when so provided on the surface of a monolithic integrated circuit. In this situation, the magnetic environment can become quite complex with fields in any one memory cell affecting the film portions in neighboring memory cells. Also, small ferromagnetic film portions in a memory cell can lead to substantial demagnetization fields which can cause instabilities in the magnetization state desired in such a cell.
These magnetic effects between neighbors in an array of closely packed ferromagnetic thin-film memory cells can be ameliorated to a considerable extent by providing a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory thin-film is provided. Such an arrangement provides significant xe2x80x9cflux closure,xe2x80x9d i.e. a more closely confined magnetic flux path, to thereby confine the magnetic field arising in the cell to affecting primarily just that cell. This result is considerably enhanced by choosing the separating material in the ferromagnetic thin-film memory cells to each be sufficiently thin. Similar xe2x80x9csandwichxe2x80x9d structures are also used in magnetic sensors.
In the recent past, reducing the thicknesses of the ferromagnetic thin-films and the intermediate layers in extended xe2x80x9csandwichxe2x80x9d structures, and adding possibly alternating ones of such films and layers, i.e. superlattices, have been shown to lead to a xe2x80x9cgiant magnetoresistive effectxe2x80x9d being present in some circumstances. This effect yields a magnetoresistive response which can be in the range of up to an order of magnitude or more greater than that due to the well known anisotropic magnetoresistive response.
In the ordinary anisotropic magnetoresistive response, varying the difference occurring between the direction of the magnetization vector in a ferromagnetic thin-film and the direction of sensing currents passed through that film leads to varying effective electrical resistance in the film in the direction of the current. The maximum electrical resistance occurs when the magnetization vector in the field and the current direction therein are parallel to one another, while the minimum resistance occurs when they are perpendicular to one another. The total electrical resistance in such a magnetoresistive ferromagnetic film can be shown to be given by a constant value, representing the minimum resistance, plus an additional value depending on the angle between the current direction in the film and the magnetization vector therein. This additional resistance has a magnitude characteristic that follows the square of the cosine of that angle.
Operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector in such a film portion with respect to the easy axis of that film. Such an axis comes about in the film because of an anisotropy therein typically resulting from depositing the film during fabrication in the presence of an external magnetic field oriented in the plane of the film along the direction desired for the easy axis in the resulting film. During subsequent operation of the device having this resulting film, such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the film magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the film""s easy axis. The state of the magnetization vector in such a film can be measured, or sensed, by the change in resistance encountered by current directed through this film portion. This arrangement has provided the basis for a ferromagnetic, magnetoresistive anisotropic thin-film to serve as a memory cell.
In contrast to this arrangement, the resistance in the plane of a ferromagnetic thin-film is isotropic for the giant magnetoresistive effect rather than depending on the direction of the sensing current therethrough as for the anisotropic magnetoresistive effect. The giant magnetoresistive effect involves a change in the electrical resistance of the structure thought to come about from the passage of conduction electrons between the ferromagnetic layers in the xe2x80x9csandwichxe2x80x9d structure, or superlattice structure, through the separating nonmagnetic layers with the resulting scattering occurring at the layer interfaces, and in the ferromagnetic layers, being dependent on the electron spins. The magnetization dependant component of the resistance in connection with this effect varies as the sine of the absolute value of half the angle between the magnetization vectors in the ferromagnetic thin-films provided on either side of an intermediate nonmagnetic layer. The electrical resistance in the giant magnetoresistance effect through the xe2x80x9csandwichxe2x80x9d or superlattice structure is lower if the magnetizations in the separated ferromagnetic thin-films are parallel and oriented in the same direction than it is if these magnetizations are antiparallel, i.e. oriented in opposing or partially opposing directions. Further, the anisotropic magnetoresistive effect in very thin films is considerably reduced from the bulk values therefor in thicker films due to surface scattering, whereas a significant giant magnetoresistive effect is obtained only in very thin films. Nevertheless, the anisotropic magnetoresistive effect remains present in the films used in giant magnetoresistive effect structures.
A memory cell based on the xe2x80x9cgiant magnetoresistive effectxe2x80x9d can be provided by having one of the ferromagnetic layers in the xe2x80x9csandwichxe2x80x9d construction being prevented from switching the magnetization direction therein from pointing along the easy axis therein in one to the opposite direction in the presence of suitable externally applied magnetic fields while permitting the remaining ferromagnetic layer to be free to do so in the same externally applied fields. In one such arrangement, a xe2x80x9cspin-valvexe2x80x9d structure is formed by providing an antiferromagnetic layer on the ferromagnetic layer that is to be prevented from switching in the externally applied fields to xe2x80x9cpinxe2x80x9d its magnetization direction in a selected direction. In an alternative arrangement often termed a xe2x80x9cpseudo-spin valvexe2x80x9d structure, the ferromagnetic layer that is to be prevented from switching in the externally applied fields is made sufficiently thicker than the free ferromagnetic layer so that it does not switch in those external fields provided to switch the free layer.
Thus, a digital data memory cell based on the use of structures exhibiting the giant magnetoresistive effect is attractive as compared to structures based on use of an anisotropic magnetoresistive effect because of the larger signals obtainable in information retrieval operations with respect to such cells. Such larger magnitude signals are easier to detect without error in the presence of noise thereby leading to less critical requirements on the retrieval operation circuitry.
An alternative digital data bit storage and retrieval memory cell suited for fabrication with submicron dimensions can be fabricated that provides rapid retrievals of bit data stored therein and low power dissipation memory through use of a cell structure that has a spin dependent tunneling junction (SDTJ), or magnetoresistive tunnel junction (MTJ), device therein based on a pair of ferromagnetic thin-film layers having an electrical insulator layer therebetween of sufficient thinness to allow tunneling currents therethrough. This memory cell can be fabricated using ferromagnetic thin-film materials of similar or different kinds in each of the magnetic memory films present in such a xe2x80x9csandwichxe2x80x9d structure on either side of an intermediate nonmagnetic layer where such ferromagnetic films maybe composite films, but this intermediate nonmagnetic layer conducts electrical current therethrough based primarily on the quantum electrodynamic effect xe2x80x9ctunnelingxe2x80x9d current mentioned above.
This xe2x80x9ctunnelingxe2x80x9d current has a magnitude dependence on the angle between the magnetization vectors in each of the ferromagnetic layers on either side of the intermediate layer due to the transmission barrier provided by this intermediate layer depending on the degree of matching of the spin polarizations of the electrons tunneling therethrough with the spin polarizations of the conduction electrons in the ferromagnetic layers, the latter being set by the layer magnetization directions to provide a xe2x80x9cmagnetic valve effectxe2x80x9d. Such an effect results in an effective resistance or conductance characterizing this intermediate layer with respect to the xe2x80x9ctunnelingxe2x80x9d current therethrough. In addition, an antiferromagnetic layer against one of the ferromagnetic layers is used in such a cell to provide different magnetization switching thresholds between that ferromagnetic layer and the other by fixing, or xe2x80x9cpinningxe2x80x9d, the magnetization direction for the adjacent ferromagnetic layer while leaving the other free to respond to externally applied fields. Such devices may be provided on a surface of a monolithic integrated circuit to thereby allow providing convenient electrical connections between each such memory cell device and the operating circuitry therefor.
Such a xe2x80x9csandwichxe2x80x9d structure in this kind of a memory cell that is relatively long in the Cartesian coordinate y with respect to its width in the Cartesian coordinate x, and based on having an intermediate thin layer of a nonmagnetic, dielectric separating material with two major surfaces on each of which a anisotropic ferromagnetic thin-film is positioned with an induced easy axis parallel to the width, exhibits the xe2x80x9cmagnetic valve effectxe2x80x9d if the materials for the ferromagnetic thin-films and the intermediate layers are properly selected and have sufficiently small thicknesses. The resulting xe2x80x9cmagnetic valve effectxe2x80x9d can yield a response which can be several times in magnitude greater than that due to the xe2x80x9cgiant magnetoresistive effectxe2x80x9d in a similar sized cell structure.
An approximation to determine the cell SDT junction switching magnetic field threshold and associated current for a cell small enough to have single domain ferromagnetic layers can be obtained from minimizing the cell free energy which is equivalent to setting the magnetic torque, Tq1, on the free layer to zero for that layer having a magnetic saturation of Ms, or
0=Tq1=xe2x88x92MsHk1 sin xcex81 cos xcex81+MsHsSf1 sin xcex81xe2x88x92MsHcp sin xcex81+MsHdemagx1 cos xcex81 sin xcex81+MsHw cos xcex81xe2x88x92MsHdemagy1 cos xcex81.
In this equation,
Hdemagx1 is the maximum demagnetizing field across the width of the cell junction free layer with Hdemagx1=4xcfx80MsT/(T+W) where T is the free layer thicknesses and W is the cell junction width in Angstroms;
Hdemagy1 is the maximum demagnetizing field across the width of the cell junction free layer with Hdemagy1=RHdemagx1 where R is the ratio of the demagnetization field along the layer to that across the layer determined by the cell junction shape;
Hk1 is the effective anisotropy field;
Hs is the maximum value of the sense field (at outer edges, i.e. the layer outer major surfaces) provided by a sense current through the cell parallel to the cell barrier;
Sf1 is the fraction of the maximum sense field that characterizes the average sense field through the thickness of a layer calculated from the layer conductivity;
Hw is an externally applied magnetic field along the cell junction length resulting from a current established in a conductor (word line);
Hcp is the effective coupling field arising from facing ferromagnetic layers uneven surface textures, or xe2x80x9corange peelxe2x80x9d effect, with the coupling taken as being from the xe2x80x9cpinnedxe2x80x9d layer to the free layer; and
xcex81 is the angle of the layer magnetization away from the easy axis that is across the width of the cell junction and antiparallel to the magnetization direction of the xe2x80x9cpinnedxe2x80x9d layer absent applied fields.
Thus, after substitution, the foregoing equation can be rewritten as       0    =                  T        q1            =                                    -                          M              s                                ⁢                      H            k1                    ⁢          sin          ⁢                      xe2x80x83                    ⁢                      θ            1                    ⁢          cos          ⁢                      xe2x80x83                    ⁢                      θ            1                          +                              M            s                    ⁢                      H            s                    ⁢                      S            f1                    ⁢          sin          ⁢                      xe2x80x83                    ⁢                      θ            1                          -                              M            s                    ⁢                      H            cp                    ⁢          sin          ⁢                      xe2x80x83                    ⁢                      θ            1                          +                              M            s                    ⁢          12500          ⁢                      T                          T              +              W                                ⁢          cos          ⁢                      xe2x80x83                    ⁢                      θ            1                    ⁢          sin          ⁢                      xe2x80x83                    ⁢                      θ            1                          +                              M            s                    ⁢                      H            w                    ⁢          cos          ⁢                      xe2x80x83                    ⁢                      θ            1                          -                              M            s                    ⁢                      H            demagy1                    ⁢          cos          ⁢                      xe2x80x83                    ⁢                      θ            1                                ,
thereby showing the layer geometry dependence. The switching threshold can be found from this latter equation by increasing the magnetic fields in small increments until the magnetization of the free layer switches to the opposite direction, and from the values of these fields at that point the currents to provide them can be determined.
As stated above, operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector with respect to the easy axis in the ferromagnetic films of these various kinds of memory cell devices. Such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the film magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the film""s easy axis, the state of the cell determining the value of the binary bit being stored therein. One of the difficulties in such memories is the need to provide memory cells therein that have extremely uniform switching thresholds and adequate resistance to unavoidable interjected magnetic field disturbances in the typical memory cell state selection scheme used. This scheme is based on selective externally imposed magnetic fields provided by selectively directing electrical currents over or through sequences of such cells so that selection of a cell occurs through coincident presences of such fields at that cell.
Such a coincident interjected magnetic fields memory cell state selection scheme is very desirable in that an individual switch, such as that provided by a transistor, is not needed for every memory cell, but the limitations this selection mode imposes on the uniformity of switching thresholds for each memory cell in a memory make the production of high yields difficult. The stability of data in memory cells in magnetoresistive random access memories (MRAM) can be improved by using circuit switching selection of the cells rather than word line and sense line coincident current selection of the cells. One memory cell structure for such an arrangement has a spin dependent tunneling junction (SDTJ), or magnetoresistive tunnel junction (MTJ), device, based on a pair of ferromagnetic thin-film layers having an electrical insulator layer therebetween of sufficient thinness to allow tunneling currents therethrough, but with one of these ferromagnetic layers effectively replaced by a substituted xe2x80x9csandwichxe2x80x9d structure formed with an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory thin-film is provided.
Such a xe2x80x9csandwichxe2x80x9d structure can be formed, for instance, as an anisotropic magnetoresistive (AMR) effect device in which the electrical resistance is at least partly determined by the angle between the direction of current therethrough and the direction of film magnetization or as a giant magnetoresistive (GMR) effect device in which the electrical resistance is at least partly determined by the angle between the directions of the two film magnetizations. That is, in this latter situation for example, one of the ferromagnetic thin-film layers in the spin dependent tunneling device is formed, instead of by a single or stratified ferromagnetic layers, a pair of sufficiently thin ferromagnetic thin-film layers separated by a sufficiently thin nonmagnetic material layer such as an electrical conductor to result in an integrated GMR tunneling device memory cell, a cell in which both integrated devices are typically formed several times longer than wide.
Such an arrangement allows for the use of much smaller electrical currents in forming magnetic fields to cause the storing of data bits in the cell so formed because substituted xe2x80x9csandwichxe2x80x9d structures take less current to have the magnetizations of the ferromagnetic thin-film layers therein switched from one magnetization state to the opposite state than would a single or stratified ferromagnetic thin-film layer. One contributing factor to this result is the magnetic flux path closure provided by the two ferromagnetic thin-film layers in the substitute xe2x80x9csandwichxe2x80x9d structure leading to reduced demagnetization fields in those ferromagnetic layers to be overcome in switching the magnetization states of such ferromagnetic layers therein.
A similar approximation to that above can again be used to determine the resulting cell substitute xe2x80x9csandwichxe2x80x9d structure switching magnetic fields threshold and associated currents by again minimizing the device free energy for a cell small enough to have single domain cell substitute xe2x80x9csandwichxe2x80x9d structure ferromagnetic layers. Such a minimization is equivalent to setting the magnetic torques, Tq1 (layer closest to junction) and Tq2 (layer further from junction), on the substitute xe2x80x9csandwichxe2x80x9d structure ferromagnetic layers each to zero where each layer has a magnetic saturation Ms, or
0=Tq1=xe2x88x92MsHk1 sin xcex81 cos xcex81+MsHsSf1 sin xcex81xe2x88x92MsHcp sin xcex81+MsHdemagx1 sin xcex81+MsHw cos xcex81xe2x88x92MsHdemagy1 cos xcex81,
and
0=Tq2=xe2x88x92MsHk2 sin xcex82 cos xcex82+MsHsSf2 sin xcex82+MsHdemagx2 sin xcex82+MsHw cos xcex82xe2x88x92MsHdemagy2 cos xcex82.
In these equations,
Hdemagx1 and Hdemagx2 are the effective demagnetizing fields across the widths of the corresponding ones of the cell substitute xe2x80x9csandwichxe2x80x9d structure layers with Hdemagx1=Hdx1 cos xcex81xe2x88x92Hdx2 cos xcex82 and with Hdemagx2=Hdx2 cos xcex82xe2x88x92Hdx1 cos xcex81 where Hdx1=4xcfx80MsT1/(T1+W) and Hdx2=47xcfx80MsT2/(T2+W) are the maximum demagnetizing fields across the widths of the corresponding ones of the cell substitute xe2x80x9csandwichxe2x80x9d structure layers which are identical for T1=T2, and Hdemagy1 and Hdemagy2 are the effective demagnetizing fields across the lengths of the corresponding ones of the cell GMR effect device layers with Hdemagy1=Hdemagy2=Hdy1 sin xcex81+Hdy2 sin xcex82 where Hdy1=RHdx1 and Hdy2=RHdx2 are the maximum demagnetizing fields across the lengths of the corresponding ones of the cell substitute xe2x80x9csandwichxe2x80x9d structure layers and R is the ratio of the demagnetization field along either layer to that across that layer as determined by the cell substitute xe2x80x9csandwichxe2x80x9d structure shape;
Hk1,2 are the effective anisotropy fields in the corresponding ones of the cell substitute xe2x80x9csandwichxe2x80x9d structure layers, and are substantially equal for the same materials being used in each layer;
Hs is the maximum value of the sense field (at outer edges, i.e. the outer major surfaces of the cell substitute xe2x80x9csandwichxe2x80x9d structure layers thereby resulting in one major surface from each layer being referenced) provided by a sense current through the cell, including through the substitute xe2x80x9csandwichxe2x80x9d structure, parallel to the cell barrier;
Sf1,2 is the fraction of the maximum sense field that characterizes the average sense field through the thickness of a corresponding one of the cell substitute xe2x80x9csandwichxe2x80x9d structure layers calculated from the layer conductivity;
Hw is a possible externally applied magnetic field along the cell length resulting from a current established in a conductor (word line) if provided at all;
Hcp is the effective coupling field arising from facing ferromagnetic layers uneven surface textures, or xe2x80x9corange peelxe2x80x9d effect, with the coupling being from the cell substitute xe2x80x9csandwichxe2x80x9d structure layer further from the junction to the layer closer thereto; and
xcex81,2 are angles of corresponding ones of the cell substitute xe2x80x9csandwichxe2x80x9d structure layer magnetizations away from the easy axes that are across the widths of those layers and antiparallel to one another absent applied magnetic fields.
Again, the switching threshold can be found from these equations by increasing the magnetic fields in small increments until the magnetization of the free layer switches to the opposite direction, and from the values of these fields at that point the currents to provide them can be determined. These threshold currents will be significantly less than those found above for the cell junction device alone because the maximum demagnetization fields for the cell substitute xe2x80x9csandwichxe2x80x9d structure layers in place of the single free layer in the cell junction device will be so much less due to the flux path closure provided by the cell substitute xe2x80x9csandwichxe2x80x9d structure layers, these two equally thick ferromagnetic layers about the nonmagnetic intermediate layer of thickness g having a joint demagnetization field of approximately 16xcfx80MsTg/W2. Nevertheless, there is a strong desire to further reduce the operating currents needed with such a memory cell.
The present invention provides a ferromagnetic thin-film based digital memory including a memory cell having a bit structure with a nonmagnetic intermediate layer exhibiting two major surfaces on opposite sides thereof and a memory film of an anisotropic ferromagnetic material on each of the intermediate layer major surfaces with a film thickness difference there of at least five percent, or a film effective anisotropy field difference because of different ferromagnetic materials used therefor, or both. An electrically insulative intermediate layer is provided on the memory film and across said memory film from one of the intermediate layer major surfaces, this intermediate layer having a major surface on a side opposite the memory film on which a magnetization reference layer is provided having a relatively fixed magnetization direction. Manipulation circuitry has a plurality of transistors coupled to the bit structure that selectively substantially prevents current in at least one direction along a current path through that bit structure, and includes switching transistors to permit selecting the occurrence of current flow through the bit structure if to be permitted at a selected time.